Otto Rethel was a German history, genre and portrait painter.

Rethel, the younger brother of the well-known history painter Alfred Rethel studied in Aachen from 1840 to 1842 at the Düsseldorf Art Academy with Karl Ferdinand Sohn and Wilhelm von Schadow. He then specialized in biblical motifs and larger altarpieces until the 1850s.

When his brother Alfred became increasingly ill with a brain disease with associated dementia from 1852, Rethel supported him and after his death in 1859 also took care of the young widow of his brother, Marie, daughter of the painter August Grahl. These stressful experiences seem to be the trigger for his change from biblical motifs to the genre images. Art critics particularly appreciated the expressive portrayal of personality and the color tones of the flesh in the portraits.

Rethel worked all his life in Düsseldorf and was an active member of the artists 'association Malkasten.

Around 1850 Otto Rethel lived with his mother Johanna and his unmarried sister Emma on Kastanienallee 297. In September 1850 he married the musician Emma (1824-1882), born Haldensleben from Brühl, moved in the Jägerhofstraße Haus Nr. 27 and 1870 in the house no. 1 on the second floor of the courtyard house. The painter Philipp Röth also lived there around 1870. Around 1872 he bought a small house on Rosenstrasse 38. He lived there until his death in 1892. Rethel was buried in the northern part, his brother Alfred and his mother Johanna in the southern part of the historic Golzheim cemetery.